The present invention regards a cement-based photocatalytic composition, and the use thereof for obtaining water paints, in particular for outdoor applications.
Photocatalysis is a natural phenomenon that regards some substances, known as photocatalysts, which—when irradiated with light of suitable wavelength—are capable of catalyzing some chemical reactions. In particular, in the presence of air and light, oxidative processes are activated on a surface containing a photocatalytic substance that lead to the transformation and/or decomposition of organic and inorganic polluting substances (microbes, nitrogen oxides, polycondensate aromatic products, benzene, sulfur dioxide, carbon monoxide, formaldehyde, acetaldehyde, methanol, ethanol, benzene, ethylbenzene, methylbenzene, nitrogen monoxide and dioxide). Such polluting and/or toxic substances are transformed, through the photocatalysis process, into innocuous substances that can be washed away by rain water or via washing, such as sodium nitrate (NaNO3), calcium sulfate (CaSO4), calcium nitrate (Ca(NO3)2) and calcium carbonate (CaCO3).
Photocatalytic processes can then be used for considerably reducing the pollutants present in the environment, such as those produced by the exhaust gases of automobiles, factories, home heating and other sources, and at the same time eliminate dirt, mold, and bacteria that degrade the external surfaces of buildings or other structures.
The photocatalysts are generally metal compounds such as titanium dioxide, TiO2, the most active and most used, zinc oxide, ZnO, and other oxides and sulfides (CeO2, ZrO2, SnO2, CdS, ZnS, etc.).
Much effort has been expended to provide compositions containing a photocatalyst to be used for coating building surfaces, which can be applied with the means commonly employed in the building industry; such compositions ensure a significant and enduring photocatalytic effect, simultaneously ensuring a satisfactory aesthetic effect, as well as of course at non-excessive costs, so as to allow the application thereof on a large scale.
According to the prior art, the photocatalytic product is usually incorporated in formulations of paints or varnishes with substantially organic base of conventional type. Nevertheless, such formulations, given that they are of organic nature, undergo the action of transformation and/or decomposition catalyzed by the photocatalyst, so that the properties of the applied coating are degraded over time, with detachment and pulverization phenomena, as well as causing a quick decay of the original photocatalytic properties.
Also known in the art are cement-based compositions which comprise a photocatalyst.
For example, in the patent application WO 2009/013337, photocatalytic compositions are described which comprise: a hydraulic binder; a polycarboxylic or acrylic superfluidizing agent; a cellulose ether with viscosity comprised between 10,000 and 120,000 mPa·s; an adhesive agent; a calcareous, silicic or silicocalcareous filler; a photocatalyst. Such compositions would be provided with rheological properties such to render them particularly suitable for the application on large surfaces, without dripping or deformations.
In the patent application WO 2013/018059, a photocatalytic powder paint is described for use diluted in water, which comprises: Portland cement combined with photocatalytic titanium dioxide in nanoparticle form; a calcareous inert substance with maximum particle size lower than 100 μm; cellulose with viscosity lower than 1000 mPa·s; a fluidizing agent; an anti-foaming agent; a vinyl polymer; pigments. Such composition also comprises at least one of the following additives: metakaolin, calcium formate and diatomaceous earth.